It is well recognized in the railway industry that, either through manufacturing processes or through natural environmental processes and normal use, rails of a railroad track develop certain detrimental flaws. The flaws may include both non-critical and critical defects, for example, transverse defects, vertical sheer or split head defects, and horizontal sheer or split head defects.
Safe operation on a rail may continue as long as the flaws remain non-critical. However, in time, even non-critical flaws may abscess or degrade into critical defects, and new flaws will arise. If the flaws are left unattended, the resulting defects could lead to a range of problems including catastrophic failure and train derailment. Catastrophic or even lesser failures present a financial, health and safety risk to the railway industry, transported goods and personnel, as well as surrounding homes and businesses. Such failures can be prevented, or at least decreased in frequency through routine inspection and maintenance. Further, routine maintenance can be made more cost-effective through selective repair, which is in turn facilitated by flaw detection.
To detect rail flaws or defects, ultrasonic testing has been employed. Vehicles and rail car-mounted inspection apparatus have been built to travel along the track and continuously perform ultrasonic inspection of the rails of the railroad track in situ.
In general, the inspection apparatus has taken the form of carriage-mounted sleds and wheels. A sled design, such as shown in U.S. Pat. No. 4,700,574, employs an ultrasonic transducer on a sled that is pulled along a rail coated with water to create an acoustic bridge. A wheel design, such as shown in U.S. Pat. No. 6,055,862, employs an ultrasonic transducer contained within a small, thin-walled tire that rolls along a rail. The tire is filled with fluid, such as a water-antifreeze solution, to create an ultrasonic bridge between the transducer and rail.
The prior art designs have a number of disadvantages. For example, sled designs require a large amount of water for adequate sled-to-rail coupling and reduced wear from long-distance travel. Additionally, sleds are more sensitive to imperfections and flaws on the running surface of the rail. Wheel designs suffer from acoustic reverberation (noise) caused by acoustic reflections from the surface walls of the tire. The noise reduces the sensitivity of the apparatus following each acoustic emission.
Further, both designs suffer from the limitations of known transducer technologies and configurations. Some acoustic transducer designs emit ultrasonic beams in a substantially vertical direction, which identifies some flaws in the web of the rail while lacking sensitivity to flaws in the internal portions of the head of the rail. In other designs, such as U.S. Pat. No. 4,700,574 and U.S. Pat. No. 6,055,862, transducers are positioned to emit ultrasonic beams at angles to the transverse and longitudinal planes of the rail. The axis of the beam intersects the rail at one side of the central vertical longitudinal plane of the rail and extends across the longitudinal plane of the rail to the other side. The designs identify some additional flaws in the rail, particularly in the side of the head of the rail. Still other acoustic transducer designs include more elaborate transducer assemblies with additional beam paths, but still have limited ability to identify flaws in the internal portion of the head of the rail.